[2 Mark Question Answer]

Question 12 Marks

If either of the displacement or the force applied changes, how does the amount of work done get affected ?

Answer

Amount of work done is directly proportional to both the displacement as well as the force applied. So, work done increases if either of the displacement or the force applied increases and decreases if either of the displacement or the force applied decreases.

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Question 22 Marks

Explain why water stored in a dam has potential energy.

Answer

The water is stored in a reservoir at a certain height above a dam. As gravitational potential energy is the possessed by an object due to its raised position, therefore, water in a dam possesses gravitational potential energy.

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Question 32 Marks

How can you measure the gravitational potential energy of an object?

Answer

The gravitational potential energy of an object can be measured by the expression $P.E. = mgh$
where, $m =$ the mass of the object
$g =$ acceleration due to gravity
$h =$ height at which the object is placed above the reference level.

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Question 42 Marks

State two differences between power and energy.

Answer

Energy :
It is the capacity to do work.
It doesn’t depend on time.
Power :
It is the amount of energy transferred in unit time.
It depends on time.

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Question 52 Marks

Two bodies of equal masses move with uniform velocities $v$ and $3\ v,$ respectively. Find the ratio of their kinetic energies.

Answer

For body $1$,
Mass $= m$
Velocity $= v$ Kinetic energy, $(K.E.)_1=\frac{1}{2} m v^2$
For body 2, Mass $= m$
Velocity $=3 v$
Kinetic energy, $(K.E.)_2=\frac{1}{2} m \times(3 v )^2=\frac{9}{2} m v ^2$
Ratio of their kinetic energies $=\frac{( K . E )_1}{( K E)_2}=\frac{1 / 2 m v^2}{1 / 9 m v^2}=\frac{1}{9}$

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Question 62 Marks

A $900\ kg$ compact car is moving at a certain speed. If its kinetic energy is $3,20,000\ J,$ find the velocity at which the car is travelling.

Answer

Mass of the car, $m = 900 \ kg$
Velocity of the car, $v =$ ?
Kinetic energy of the car, $K.E. = 3,20,000 J$
From the formula, $ K.E.=\frac{1}{2}\ m v^2$
We get, $3,20,000\ J =\frac{1}{2} \times(900 \ kg ) v ^2$
$v ^2=711.11 m ^2 / s ^2$
or $v = 26.67 m/s$

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Question 72 Marks

Calculate the work done by a boy in lifting a to $kg$ laptop from the ground and keeping it on a shelf $1.5 \ m$ high.

Answer

Mass of the laptop$, m\ – 10 \ kg$
Height of the shelf where laptop is placed$, h\ – 1.5\ m$
$g = 10\ m/s^2$
Potential energy of the laptop$, P.E. = mgh$
$= (10 \ kg) \times (10\ m/s^2) \times (1.5\ m)$
$= 150\ J$
From work$-$energy relationship, we can deduce that the work done by the boy in lifting the laptop gets stored in it as gravitational potential energy. Hence, Work done in lifting the laptop $=$ Energy change in the laptop $= 150\ J$

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Question 82 Marks

If $1000\ J$ of work is done by a machine to move a body through a distance of $20\ m$, then find the force applied on the body.

Answer

Work done by a machine on a body, $W = 1000\ J$
Displacement of the body, $s = 20\ m$
Force applied on the body, $F =$ ?
Using the formula,
$W = F \times s$
$1000\ J = F \times 20\ m$
$F =\frac{1000 J }{20 m }=50\ N$

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Question 92 Marks

A child climbs a wall of height $4\ m$. He now possesses a potential energy of $1600\ J$. What is the mass of the child?

Answer

Mass of the child$, m = ?$
Height of the child$, h = 4\ m$
$g = 10\ m/s^2$
Potential energy of the book with respect to the floor$, P.E. = 1600\ J$
From the formula,
$P.E. = mgh$ or $1600$
$J = m \times (10\ m/s^2) \times (4\ m)$
or $m = 40\ kg$

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Question 102 Marks

A motorbike moving at a speed of $12 m/s$ has a kinetic energy of $3600\ J$. Determine its mass.

Answer

Mass of the motorbike, $m =$ ?
Speed of the motorbike, $v = 12\ m/s$
Kinetic energy of the motorbike, $K.E. = 3600\ J$
From the formula,
$K . E =\frac{1}{2}\ mv ^2$
$3600 \ J=\frac{1}{2} m \times(12 m / s )^2=\frac{1}{2} m \times 144 m^2 / s ^2$
$=72\ m^2 / s ^2$
$m = 50\ kg$

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Question 112 Marks

How does an archer use the law of conservation of energy to hit his target?

Answer

An archer draws back the arrow against the bowstring, the work done in stretching the string gets stored in it as elastic potential energy. When the bowstring is released, the stored potential energy is transformed into the kinetic energy of the string. The motion of the string launches the arrow on to the target.

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PHYSICS [2 Mark Question Answer]

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